1. Field of the Invention
The present invention relates to a semiconductor device, especially a semiconductor device that accumulates electrical charges in an insulating film.
A nonvolatile semiconductor memory has been heretofore known, which includes a charge storage film formed by laminating dielectric films to be used for so-called a metal-oxide-nitride-oxide-semiconductor (MONOS) memory, and stores information by controlling the amount of electric charges accumulating in the charge storage film. The nonvolatile semiconductor memory accumulates electrical charges in charge traps discretely existing in a dielectric substance film, and this memory has robustness which makes it difficult for the electrical charges that discretely accumulate in the dielectric substance film to leak out. In other words, when the electrical charges discretely accumulate in the dielectric substance film and the electrical charges with continuous energy accumulate in a conductor, it will be more difficult for the electrical charges discretely accumulating in the dielectric substance film to leak out. For example, electrical charges accumulate in an oxide-nitride-oxide (ONO) film formed by laminating an oxide film, a nitride film, and an oxide film in a MONOS memory. Therefore, electrical charges accumulating in a nitride film will rarely leak out of the ONO film even if a thin oxide film formed between a semiconductor substrate and a nitride film has some type of defect therein.
A heretofore known nonvolatile semiconductor memory has a structure in which a gate electrode formed on a semiconductor layer as a word line is provided astride a plurality of memory cells, and a charge storage film is formed along this word line. In this type of a nonvolatile semiconductor memory, each memory cell stores 2-bit information by accumulating electrical charges in charge storage films formed on both sides of the gate electrode, respectively. In this memory structure, the gate electrode is formed on the semiconductor layer so that it lies astride a plurality of memory cells. For example, the charge storage film is an ONO film formed by sequentially accumulating an oxide film, a nitride film, and an oxide film. The oxide film and the nitride film, which comprise the lower layer of the ONO film, are formed as the sidewall portion of the gate electrode, and their cross-sections are formed to be L-shape. Also, the oxide film comprising the upper layer of the ONO film is formed to fill a portion created by the L-shape of the oxide film and the nitride film that comprise the upper layer of the ONO film.
In this type of nonvolatile semiconductor memory, electrons are injected into the charge traps existing in a silicon nitride film by controlling the voltage of a gate electrode and the voltage between the source and the drain, and thus information is stored. Electrical charges accumulate in the charge traps existing in the silicon nitride film, and those existing in the boundary surface of the silicon nitride film with a silicon dioxide film.
2. Background Information
Japanese Patent Application Publication JP-A-2003-78045 discloses another type of nonvolatile semiconductor memory that stores 2-bit information in each memory cell. In this type of the nonvolatile semiconductor memory, a first dielectric film and a gate electrode (i.e., a first memory gate electrode) are formed on a semiconductor substrate in substantially the same shape, and the surface of the gate electrode and that of a semiconductor substrate are covered with a second dielectric film. The first dielectric film and the second dielectric film are charge storage films, and each of them is an ONO film formed by sequentially laminating a silicon dioxide film, a silicon nitride film, and a silicon dioxide film. Also, a conductive sidewall portion (i.e., a second memory gate electrode) is formed on a sidewall portion of the gate electrode through the second dielectric film. This conductive sidewall portion is connected to a word line. Also, one of the source-drain regions is formed on a portion of a semiconductor substrate, which is located on either of the outer sides of the conductive sidewall portion. A first bit line is connected to this source-drain region. Also, the other source-drain region is formed on a portion of the semiconductor substrate, which corresponds to a region located from a portion immediately below the other sidewall portion to the outside of the sidewall portion. A second bit line is connected to this source-drain region.
In this type of nonvolatile semiconductor memory, 2-bit data is stored in each memory cell by a memory transistor MTa in which a channel formed below the gate electrode (i.e., the first memory gate electrode) is controlled, and a memory transistor MTb in which a channel formed below the conductive sidewall portion (i.e., the second memory gate electrode) is controlled. Electrical charges are injected into the first dielectric film and the second dielectric film, respectively, by controlling the voltage between the first bit line and the second bit line, the voltage of the memory gate electrode, and the voltage of the second memory gate electrode. Thus data storage is conducted.
In the above described memory structure in which charge storage films are formed on the sidewall portions formed on both sides of the gate electrode functioning as a word line, a nitride film has a lengthwise portion continuously formed along the sidewall of the gate electrode in the lengthwise direction, as well as a crosswise portion formed in the crosswise direction on the lower portion of the sidewall portion of the gate electrode that effectively works for charge retention. In other words, the lengthwise portion that does not function as a memory is formed in connection with the crosswise portion that functions as a memory by changing the threshold voltage Vt of a memory transistor in accordance with the storage of information. Also, the nitride film is continuously formed in a field region as well as an active region that functions as a memory element. In a MONOS memory structure, electrical charges accumulate in the discrete charge traps existing in the nitride film. Therefore, electrical charges easily move even at a relatively low temperature through the charge transfer between the charge traps existing in the nitride film, although the amount of energy required for the movement of electrical charges in a nitride film is larger than that required for the movement of electrical charges in a conductor. Therefore, as described above, if the crosswise portion of the nitride film that functions as a memory is connected to the lengthwise portion of the nitride film that does not function as a memory and the field region that does not function as a memory element, electrical charges move to the portions other than the lower side portion (i.e., the crosswise portion) of the gate electrode. As a result, a problem is caused in which the cell current occurring at the time of information readout from a memory cell changes over time.
However, the changes in cell current over time caused by the movement of electrical charges in the nitride film is not described in Japan Patent Application Publication JP-A-2003-78045.